In recent years this species has been confused with M. microcephala H.B.K., principally because further material from Guiana has shown wider variability in the distinguishing foliar characters than was allowed in the original descriptions.

In general, M. microcephala is found in Edo. Bolivar and Terr. Amazonas, Venezuela, within the phytogeographical province of Guayana, occurring as an unarmed shrub of savannas. The leaves have 12-41 pairs of pinnae, the pinnae with 20-45 pairs of leaflets: the leaflets are 1.5-3.5 mm long, and the pod is 3-6-seeded.

M. plumifolia is an armed subscandent shrub of more or less permanently open places in the forest (large outcrops, margins of rapids, rocky river islands, etc.) in Guiana, and until recently has been known only from the interior of Suriname. This species bears leaves with 8-18 pairs of pinnae, pinnae with 35-50 pairs of leaflets; the leaflets are 4-6 mm long, and the pod is 7-10-seeded.

As may be seen from this comparison of critical characters, the only clear distinction between Guayana and Guiana populations has been the presence or absence of thorns and the number of seeds in the pod. Hitherto the maintenance of two separate taxa has been reinforced by geographical disjunction; to date neither is known to occur in British Guiana, although either may be expected there. In Steyermark 89266, however, there is almost complete agreement with the characters given by Kleinhoonte for the Surinam plant. A further indication of the complexity of the problem is seen in Ducke 1607, taken at Sao Marcos, Terr, do Roraima, an unarmed savanna plant approaching M. plumifolia in several respects.